In recent years, chemotherapy and radiation therapy of patients with cancer has been increasingly successful and sustained remissions have been achieved. However, the chronic side effects of these therapies to the reproductive systems of long-term survivors is of particular concern. These effects for women include depletion of ovarian germ cells and sterility. Due to the potential loss of future fertility of those exposed to cancer therapy, a need for oocyte banking has developed. Oocyte freezing, when combined with in vitro fertilization, may be beneficial to women desiring future fertility who are anticipating loss of gonadal function from extirpative therapy, radiation, or chemotherapy. Oocyte freezing may also provide a possible alternative to human embryo freezing, thus avoiding many of the legal and ethical problems encountered in embryo freezing.
The first successful cryopreservation of human embryos was achieved in 1983 and embryo freezing is now a routine procedure. In contrast, very limited success has been reported with cryopreservation of human oocytes. Only five successful pregnancies have been reported with more than 1500 cryopreserved oocytes. Therefore, the current methods of freezing are still considered experimental and novel approaches are needed to overcome the difficulty encountered by cryopreservation of the human oocyte.
Traditional cryopreservation techniques include penetrating cryoprotectants at concentrations of 1 to 2 M with, for example, dimethyl sulfoxide (DMSO), glycerol, or ethylene glycol, followed by a slow freezing rate (0.3 to 0.5° C./min). Typically, oocytes are damaged due to long-term exposure to deleterious freezing conditions, including excessive dehydration and high electrolyte concentrations. An alternative approach, called vitrification (i.e., formation of glassy material without crystallization of ice, uses high concentrations of cryoprotectant mixtures (6 to 8 M) followed by rapid cooling in order to avoid the lethal effects of freezing on oocytes.
Though an attractive alternative, vitrification procedures suffer from the toxic and osmotic effects of high cryoprotectant concentration on sensitive cells. Neither of these two approaches (slow freeze-thaw and rapid vitrification) has resulted in a reliably successful outcome for cryopreservation of human oocytes. Thus, there is a need for a reliable technique for human oocyte storage. In order to provide the preservation of mammalian cells necessary for application of living cells as a therapeutic tool in clinical medical care, new protocols for preserving living nucleated cells using low levels of non-toxic preservation agents and having simple procedures applicable to a variety of cells must be developed.